The invention relates to a method for extending the useful life of a rechargeable energy storage unit, such as, in particular, a double layer capacitor, a lithium ion storage unit, a lithium ion capacitor or a NiMH storage unit, which exhibits a plurality of storage cells.
Rechargeable energy storage units such as, in particular, double layer capacitors, lithium ion storage units, lithium ion capacitors and NiMH storage units, usually have a plurality of storage cells, which are connected in series. Production variations result in a scattering of the physical properties of the storage cells, a feature that, among other things, has an impact on their useful life. Therefore, storage cells, which are actually identical, vary, for example, with respect to their storage capacity and their internal resistance. This feature in turn leads to a fluctuation in the increase in temperature at the same load. It is known that the useful life of a storage cell decreases as the cell voltage increases and as the temperature increases.
The object of the invention is to provide measures for extending the useful life of a rechargeable energy storage unit.
An important first aspect of the invention consists of measuring the temperature of each of the storage cells. If the temperature of a storage cell exceeds a reference temperature, the voltage of the storage cell is reduced by a targeted negligible discharge of this storage cell. The targeted reduction in the voltage of the storage cell results in an increase in the average useful life of the storage cell in question and obviates the reduction of the average useful life of the storage cell as a consequence of its higher temperature. Preferably the voltage of the storage cell in question is reduced just so far that the accompanying extension of the useful life largely compensates for the reduction in the useful life owing to the raised temperature.
An important second aspect of the invention consists of measuring the capacity of each of the storage cells. If the capacity of a storage cell falls below a reference value for the capacity, the voltage of the storage cell is reduced by a targeted negligible discharge of this storage cell. The targeted reduction in the voltage of the storage cell causes an increase in the average useful life of the storage cell in question and obviates the reduction in the average useful life of the storage cell owing to its lower capacity. Preferably the voltage of the storage cell in question is reduced just so far that the accompanying extension of the useful life largely compensates for the reduction in the useful life owing to a capacity that is too low.
It is clear that a plurality of storage cells may be connected together to form a storage module. Then a plurality of storage modules form the rechargeable energy storage unit. Optionally it may be expedient to apply the measures of the invention to the storage modules and not to each storage cell.
The measures of the invention (either individually or in combination) make it possible to significantly increase the useful life of the rechargeable energy storage unit. The useful life of the series connected storage cells is defined by the storage cell, whose useful life is the shortest. Therefore, the useful life of the energy storage unit can be extended the most effectively by extending the useful life of the storage cell that exhibits the shortest life in the event that the operating temperature is too high and/or by reducing the operating voltage of the storage cell in the event that the capacity is too low.
It is also especially important that the invention also provides measures against the ageing of the storage cells. The ageing, induced by years of operation, may significantly increase the internal resistance of the individual storage cells and/or dramatically decrease their capacity, as a result of which the operating temperature rises. If no countermeasures are applied, the ageing of these storage cells may even accelerate, so that the end result is the total failure of the rechargeable energy storage unit in situations, in which the storage cell in question (that is a part of a series connection) fails.